The present application relates to a non-aqueous electrolytic solution secondary battery, and in particular relates to a non-aqueous electrolytic solution secondary battery where a winding group, where a positive electrode having a positive electrode collector applied with a positive electrode mixture including a positive electrode active material from/in which lithium-ions can be released/occluded through charging/discharging and a negative electrode having a negative electrode collector applied with a negative electrode mixture including a negative electrode active material in/from which lithium-ions can be occluded/released through discharging/charging are wound though a separator, is infiltrated into non-aqueous electrolytic solution where a lithium salt is dissolved into organic solvent, and is accommodated into a battery container.
Conventionally, in a lithium secondary battery using a metallic lithium or a lithium alloy for a negative electrode, there has been a drawback in that internal shorts between a positive electrode and the negative electrode occur since a dendritic lithium deposits on the negative electrode at the time of charging. In order to solve this problem, a lithium-ion secondary battery using a lithium transition metallic complex oxide such as lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4) or the like as a positive electrode active material, and a carbon material as a negative electrode active material, has been developed recently. Among the lithium transition metallic complex oxides, a complex oxide using manganese has an advantage over a complex oxide using cobalt in cost performance due to its abundant quantity of natural resources, and it also has an advantage in safety.
Because the lithium-ion secondary battery has high energy density as its merit, attention to the battery is being paid not only as a power supply source for portable equipment mainly such as a VTR camera, a note type computer, a portable telephone and the like, but also as a power supply source mounted for a vehicle. Namely, in order to cope with the environmental problems in the automotive industry, development of an electric vehicle (hereinafter called xe2x80x9cEVxe2x80x9d) whose power source is confined entirely to a battery so that there is no gas exhausting and of a hybrid electric vehicle (hereinafter called xe2x80x9cHEVxe2x80x9d) where both an internal combustion engine and a battery are used as its power source, has been accelerated, and some of them have reached a practical stage.
The lithium-ion secondary batteries can be classified into cylindrical shape ones and rectangular shape ones according to their outer shapes. The cylindrical shape ones are popular at the present time. The interior of the cylindrical lithium-ion secondary battery has a winding type structure where each of the positive electrode and the negative electrode comprises a metallic foil applied with an active material and is formed in a strip shape, and to form a winding group the positive and negative electrodes are wound spirally through a separator so as not to directly come in contact with each other. The winding group is accommodated in a cylindrical battery can which is a battery container, and after the battery container is filled with non-aqueous electrolytic solution, it is sealed and then by initial charging, the cylindrical battery is given a function as a secondary battery.
The battery for the power supply source of the EV/HEV is required to have high power and high-energy characteristics. In order to meet the requirement, various contrivances are needed not only to the winding group but also to the non-aqueous electrolytic solution. As a contrivance example for the non-aqueous electrolytic solution, Applicant disclosed a technique, in a Japanese patent application, serial number 11-119359, filed on Apr. 27, 1999, for setting the proportion of organic solvent to dissolve a lithium salt. As stated in the application, since generating elements such as the positive/negative electrodes and the like which comprise the winding group deteriorate extremely subject to the influence of moisture or water content, the non-aqueous electrolytic solution secondary battery adopts a hermetically sealed structure in which an opening portion of the battery container is sealed by laser-welding or the like so that the generating elements can be shut off from the atmosphere.
When the complex oxide expressed by a chemical formula LixMnyO2 (0.4xe2x89xa6xxe2x89xa61.35, 0.65xe2x89xa6yxe2x89xa61.0) is used as the positive electrode active material, there were drawbacks in that a charging/discharging cycle life characteristic of the lithium-ion secondary battery becomes short and a power (output) characteristic thereof becomes lowered. Particularly, when the battery is used at a high temperature of 50xc2x0 C. or more, there are disadvantages in that the power and cycle life characteristics deteriorate since the manganese elutes from the positive electrode and the eluted manganese forms a nonconductive (inert) coating on the surface of the negative electrode.
Further, in order to assist acceleration force of motor drive in the HEV, the charging and the discharging accompanied with a large electrical current are repeated during a short time. For this reason, in the non-aqueous electrolytic solution secondary battery for the HEV, a new battery characteristic of high outputting that has never been needed hitherto is required. A high output can be gained when the discharging is carried out at a small current in the conventional lithium-ion secondary battery, however, it had a drawback in that the output goes down remarkably when the discharging is carried out at a large current. It is considered for this reason that, since the movement of lithium-ions can not follow the rapid flowing of electrons, it causes large concentration gradient in the separator and/or electrodes. This brings about an increase in an internal resistance and results in a drop in the output.
As stated above, with progress in the development of the lithium-ion secondary battery, the battery for the EV/HEV has been required to have a higher power characteristic that the drop in the power is smaller as well as a longer life characteristic in spite of time-lapse or aging. Especially, since the HEV uses both the engine and the motor, the battery that has the high output and that can maintain high input/output characteristics, in other words, the battery where an increase in a direct current internal resistance is small even the time-lapse, is required as the power supply source for the HEV. Accordingly, it is inferred that the spread of the EV/HEV will be accelerated if such a non-aqueous electrolytic solution secondary battery with a higher power and output, and with a longer life, is obtained.
In view of the above circumstances, a first object of the present invention is to provide a non-aqueous electrolytic solution secondary battery which can prevent a drop in the power. A second object of the present invention is to provide a non-aqueous electrolytic solution secondary battery that has excellent battery characteristics at a high temperature, particularly has a high power characteristic. A third object of the present invention is to provide a non-aqueous electrolytic solution secondary battery that has high input/output characteristics. And a fourth object of the present invention is to provide a non-aqueous electrolytic solution secondary battery that has a long life characteristic.
In order to achieve the above first object, the present invention is a non-aqueous electrolytic solution secondary battery where a winding group, where a positive electrode having a positive electrode collector applied with a positive electrode mixture including a positive electrode active material from/in which lithium-ions can be released/occluded through charging/discharging and a negative electrode having a negative electrode collector applied with a negative electrode mixture including a negative electrode active material in/from which lithium-ions can be occluded/released through discharging/charging are wound through a separator, is infiltrated into non-aqueous electrolytic solution where a lithium salt is dissolved into organic solvent, and is accommodated into a battery container, wherein porosity of the separator is larger than or the same as porosity of the positive electrode mixture and porosity of the negative electrode mixture.
When the porosity of the separator is larger than the porosity of the negative electrode mixture, the separator becomes rich in the volume of the non-aqueous electrolytic solution that infiltrates into interfacial aperture between the separator and the negative electrode. Accordingly, in a case in which large current discharging is carried out in a short time, the lithium-ions are released (departed) from the negative electrode active material, and the non-aqueous electrolytic solution inside the separator can gain the released lithium-ions with a small interfacial resistance. Namely, a resistance increase according to interfacial concentration gradient can be held down. Also in the positive electrode, when the porosity of the separator is larger than the porosity of the positive mixture, since the separator becomes rich in the volume of the non-aqueous electrolytic solution that infiltrates into interfacial aperture between the separator and the positive electrode, the lithium-ions are occluded (inserted) uniformly in the positive electrode active material even in a short time. According to the present invention, since the increase in the internal resistance is held down by setting the porosity of the separator larger than or the same as the porosity of the positive electrode mixture and the porosity of the negative electrode mixture, the drop in the power at the time of the large current discharging can be controlled.
In this invention, in a case in which the positive electrode active material is a lithium-manganese complex oxide expressed by a chemical formula LixMnyO2 (0.4xe2x89xa6xxe2x89xa61.35, 0.65xe2x89xa6yxe2x89xa61.0), and porosity of the positive electrode mixture is set from 21% to 31%, since both the infiltration of the non-aqueous electrolytic solution and the diffusivity as well as conductivity of the lithium-ions are carried out adequately, a non-aqueous electrolytic solution secondary battery with a high power characteristic is realized. Therefore, the second object of the present invention can be achieved. Also, when volume of the non-aqueous electrolytic solution accommodated in the battery container is 1.0 times or more than a porosity volume of the winding group, since the non-aqueous electrolytic solution that expedites and retains an electrochemical reaction with the active materials is infiltrated and diffused into porous body formed respectively in the positive electrode mixture and the negative electrode mixture, a high initial output can be retained, and at the same time, the drop in the power as well as the increase in the direct current resistance can be suppressed. Accordingly, a non-aqueous electrolytic solution secondary battery with a high output characteristic is realized. Therefore, the third object of the present invention can be achieved.
The present invention will become more obvious by referring to the following preferred embodiments.